EFLUM post-doc's paper named a 2011-2012 highlight

The journal of Measurement Science and Technology has included an article by Marcus Hultmark, a post-doc at the Laboratory for Environmental Fluid Mechanics and Hydrology (EFLUM) in its 2011-2012 highlight edition. The article, written by Hultmark during his PhD at Princeton University, was chosen by the editorial team of the journal on the grounds of its high quality and its high rating by referees and readers. It proposes an improved criterion for the design of hot-wire anemometers - wind sensors that are commonly used to study turbulence.

For his PhD thesis, Hultmark investigated turbulence at Princeton University’s Department of Mechanical and Aerospace Engineering. “The paper is actually really simple,” he says. “We were trying to make a small sensor to measure turbulent flows when we realized that the criterion commonly used to design these sensors could be improved.” The findings led to the development of some truly tiny turbulence sensors, built at the nanometer scale.

Hot-wire anemometers work by relating the heat lost by a hot wire exposed to the wind to the windspeed. The dimensions of the wire play a critical role in determining the precision of the measurement. A wire has to be short enough to get a reliable value for a given point in space, and thin enough to respond quickly to changes in windspeed. But make it too short, and the heat lost to the structure holding the wire, rather than to the air, leads to inaccurate results. This led to the creation of a simple criterion for the design of hot wires based on the ratio between the length and the diameter of the wire.

While he was working on the design of a small sensor, Hultmark noticed that the criterion might be too simple, since it didn’t take into account the actual physics of the problem. “What we are really interested in,” he explains, “is dimensioning the sensors so that the heat lost to the support structure is negligible compared to the heat lost to the air.” By asking the right question, they were able to derive a criterion that takes into account not only the dimensions of the wire, but also its material characteristics, as well as properties of the flow that is being measured.

Although the new criterion requires more input parameters, it allows to design sensors for specific conditions, opening doors for a further miniaturization of the devices. And not only that, it also helped Hultmark and his colleagues earn the unofficial world record for the highest Reynolds number flows ever measured, with full resolution, in a lab!

Reference: 
A new criterion for end-conduction effects in hot-wire anemometry
Marcus Hultmark, Anand Ashok and Alexander J Smits
2011 Meas. Sci. Technol. 22 055401